Modern buildings are being made ever more energy efficient which often translates into better insulation and less air infiltration through cracks, gaps or the like. As a consequence, the airflow into and out of the building envelope must be very carefully managed.
Another consequence of a tighter building envelope however is the need to manage the exhaust air flow which may be required from other building systems. Three systems in particular have localized blowers to force vent air in certain circumstances, and they include bathroom exhaust fans, to vent moisture laden air from bathrooms, kitchen exhaust fans to vent the by products of cooking which may include steam, greasy vapours and smoke, and electric dryer exhausts, which again are for the purpose of venting moist air from the drying of laundry inside a dwelling or building envelope.
Particular to these types of exhaust air is the back pressure created by the local appliance, such as the bathroom exhaust fan, the kitchen exhaust fan and the electric dryer exhaust blower. Also particular to these types of exhausts is the need to let the exhaust air out when there is a need to, but to prevent air from leaking out, or more precisely to prevent outside air from leaking in or infiltrating into the building envelope, when the appliance is not in use. Concomitant with this there is a need to make the air flow passageway impervious to pests, animals, rodents and the like, that might otherwise try to gain entry into the building.
As a result, there is a need for vents which permit the air to be vented outside of the building, but which prevent unwanted air from infiltrating back into the building and which are protected from pests. Typically a flapper valve is used for this purpose which can open when the air is being exhausted and which closes when there is no air flow. A secondary need is to provide a screen or other structure which can be used to prevent animals or pests from getting into the building through the vent opening.
Various designs have been proposed in the past including vents made from metal, but these tend to be expensive and easily damaged in transit. Further a metal flapper valve can make quite a bit of noise when it opens and closes. A problem with prior art designs is that wind or weather can change the air pressure around the exterior of the vent causing the flapper to flap up and down even when the inside appliance is not turned on and there is no need for the vent to be open. This constant chatter can become a serious issue to an occupant.